The invention relates to an X-ray image intensifier tube and the radiological chain having such an intensifier.
In X-ray image intensifier tubes (IIR tubes) the incident X-rays are converted into light in a luminous screen, then into photoelectrons in a photocathode. These photoelectrons are accelerated by an electronic optics and focussed onto a luminous powder giving a luminous image of the density incident X-photon flux. For television usage this output image is taken up by an optics which re-forms the image on the photosensitive target of a camera tube, for example a Vidicon, where it creates a distribution of charges which are read by an electron beam, thus giving the video signal.
It is desirable to eliminate the IIR-Vidicon coupling optics due to its weight and overall dimensions, as well as its lack of luminosity and in general terms the additional faults which it introduces into the chain.
A first prior art solution involved IIR-Vidicon coupling by optical fibres. The luminous output screen of the IIR is brought into contact with a flat coil of optical fibres, as is the camera tube target, the two flat coils then being coupled together.
However, optical fibres have defects, which are of a serious nature when used radiologically. A defect in one of the individual fibres forming the flat coil system leads to a black zone or point and in addition the design of the fibre mosaic appears on the image.
A second prior art solution consists of eliminating the output screen of the intensifier and the optical coupling and transmitting the photoelectrons directly to a Vidicon target which is sensitive to the impact of the electrons, the system being placed in the same enclosure, such as a diode mosaic target. In this way a very high video signal--X-ray gain is obtained.
Unfortunately it is necessary to reduce to a maximum the quantumnoise of the rays by the use of very high X-doses. Moreover the dimensions of the input field of the intensifier impose high voltages on the electronic optics, which gives a high energy to the photoelectrons arriving at the target and consequently a very high electron gain in the target. Due to the high X dose and the high target gain it is necessary for the purpose of avoiding electrical saturation of the latter to provide arrangements for reducing the target gain. In such tubes it is also necessary to provide for the possibility of a target gain varying, e.g. between 1 and 50 to permit operation either in graphics or in scopics, depending on usage.
According to another prior art solution in this direction one or more thick metal barrier layers, e.g. 1 .mu.m thick aluminium, absorbing part of the energy of the electrons is deposited on a diode mosaic target on the photoelectron arrival side introduces a considerable multiplication noise due to the fact that the energy loss of the photoelectrons in the barrier layer is a statistical phenomenon having considerable fluctuations.